From a thermodynamic perspective, the ionic association may be described in terms of a chemical reaction ( Figure 1A): In comparison to that of an aqueous electrolyte solution, the relative permittivity of ILs is extremely small (∼2), implying the prevalence of ion pairing due to the strong electrostatic attraction between cations and anions ( Wang and Voth, 2005 Canongia Lopes and Pádua, 2006 Fraser et al., 2007 Niedermeyer et al., 2012). Although the structure and thermodynamic properties of ILs in the electric double layer (EDL) have been widely studied ( Chen et al., 2021a Chen et al., 2021b Gan et al., 2021 Wang et al., 2021), the dynamic behavior remains poorly understood in particular on how association between cations and anions influences charge relaxation and ion transport under confinement. Understanding their charging dynamics, i.e., the polarization of ionic distributions in response to a biased electrical potential, is desirable for many electrochemical applications ( Zhang et al., 2020 Tao et al., 2021 Tao et al., 2022). Room-temperature ionic liquids (ILs) have been broadly explored as promising electrolytes in various fields of electrochemistry ( Xiao and Johnson, 2003 Sato et al., 2004 Wang, 2020) including both batteries ( Lee et al., 2006 Rupp et al., 2008) and supercapacitors ( Ue et al., 2003 Zhou et al., 2004 Arunachalam et al., 2020 Tao et al., 2020). Analytical expressions have been obtained to reveal the quantitative relations between the dynamic timescales and the association strength.

The ionic association introduces a new decay length λ S and relaxation time scale τ R C = λ S L / D, where L is the system size and D is ion diffusivity, that are distinctively different those corresponding to non-associative systems. Here, we propose a theoretical procedure combining reaction kinetics and the modified Poisson-Nernst-Planck (MPNP) equations to study the influences of ionic association on the dynamics of electrical double layer (EDL) in response to an applied voltage. Whereas great efforts have been devoted to investigating the association effect on the equilibrium properties of ILs, a molecular-level understanding of the charging dynamics is yet to be established. 3Department of Chemical and Environmental Engineering, University of California, Riverside, Riverside, CA, United StatesĪssociation between cations and anions plays an important role in the interfacial structure of room-temperature ionic liquids (ILs) and their electrochemical performance.2Tianjin Key Laboratory for Photoelectric Materials and Devices, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin, China.1State Key Laboratory of Chemical Engineering, Frontiers Science Center for Materiobiology and Dynamic Chemistry, Shanghai Engineering Research Center of Hierarchical Nanomaterials, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai, China.Jin Cheng 1 †, Haolan Tao 1 †, Ke Ma 2 †, Jie Yang 1, Cheng Lian 1*, Honglai Liu 1* and Jianzhong Wu 3*

Charging equil note